The project portfolio of the Combustion Research Laboratory (CRL) at PSI is mainly focused on electric power generation devices which offer the highest efficiencies and lowest emissions: gas turbine based processes and internal combustion engines. The basics of relevant physical-chemical mechanisms for 'near-Zero' emission fuel conversion are studied in 5 topical areas:
Combustion Fundamentals & Technologies
Turbulent Lean Premixed Combustion
In the investigation of turbulent lean premixed CH4/air flames, further statistical analysis of OH-PLIF measurements has led to the determination of most probable flame front positions for various operating conditions relevant for gas turbine applications (pressures up to 15 bars, equivalence ratios in the range of 0.43 to 0.56). Complementary measurements of flame front tracer species (HCHO-PLIF) were performed up to 5 bars to gain more insight into the flame front structure. Derived turbulent flame speed data show (almost) no dependence of pressure.
First results with hydrogen containing mixtures (80 % CH4 and 20 % H2) indicate a significant extension of the stability limit towards leaner conditions and therefore lower combustion temperatures. NOX emissions of these CH4/ H2/air flames are considerably lower than those of pure methane/air flames.
Potential candidate technologies for ultra-low emission ("[near] zero emission") processes are being studied by the group, catalytically supported combustion being the most prominent option currently investigated. Particle Image Velocimetry (PIV) has been applied for the first time in turbulent catalytic combustion, providing direct assessment for the degree of flow laminarisation at practical Reynolds numbers (up to 40,000). Reduced hetero/homogeneous chemical reaction schemes for the combustion of fuel-lean CH4/air mixtures over platinum were developed, which were capable of reproducing key features of the corresponding full mechanisms, such as onset of homogeneous ignition. A novel and computationally efficient numerical code was developed for catalytic combustion, which employed an elliptic description for the solid phase and a parabolic one for the gas phase.
In fuel-rich catalytic combustion, Partial Catalytic Oxidation (PCO) of methane over rhodium-based catalysts was investigated as a means of producing synthesis gas for flame stabilisation in gas turbine combustors. In particular, PCO with large exhaust gas H2O and CO2) dilution was shown to be a feasible approach for nearly-zero emission power cycles.
Fuel derived radicals like CH, CHO, HCHO are better markers of the flame front region than OH, which is most often used because of its abundance. To test the applicability of HCHO as flame marker in turbulent lean premixed flames, the excitation/detection schemes best suited for laser-induced fluorescence (LIF) measurements were investigated. Systematic measurements at large & bench scale laboratory burners as well as commercial burners are other important activities of the combustion diagnostic group. OH LIF as well as OH chemiluminescence data in gas turbine burners were used to obtain information on the processes related to thermoacoustic instabilities.
Efforts are currently concentrated on the characterisation and dynamics of peroxyl radicals the key molecules of flame ignition. The design of radical sources suited to prepare ground state peroxyl radicals in a molecular beam is technically very challenging and several potential generation techniques are currently tested in our laboratory. With Four Wave Mixing spectroscopic schemes the state specific dissociation of formaldehyde to formyl can now be monitored and quantified. Theories to interpret fs-pump-probe measurements on formaldehyde are refined and generalised for their application to coming experiments with peroxyl radicals. In cooperation with the synchrotron department the Reaction Analysis group sets up an experimental facility for chemical dynamics studies. Additionally, the concept of a dedicated ultraviolet beamline at SLS is now mature and will be realised next year.
Exhaust Gas Treatment
Current focus is on selective catalytic reduction (SCR) techniques for NOx and continuously regenerated traps (CRT) for soot particles in the exhaust flow of internal combustion engines. Research activities are geared towards new catalysts with improved low-temperature activity, better selectivity and lower by-product emission risks, e.g. vanadyl species, N2O, etc. Extensive characterisation of Fe-ZSM5 catalysts gave us a deeper insight into the structure and function of this alternative, highly stable SCR catalysts. Besides good SCR performance, Fe-ZSM5 proved to be an excellent isocyanic acid hydrolysis catalyst. The search for new low-temperature SCR catalysts, revealed that MnO2-Nb2O3-CeO2 clearly exceeds the activity of vanadia based systems up to 300 °C. MnO2-CeO2 showed also a very high activity in the oxidation of Diesel soot. However, all MnO2-CeO2 based systems suffer from deactivation by sulphur due to the high stability of sulfates formed.